iMessage Problem

I just got a new iPhone to replace an older iPhone. Now, my new phone is not talking to my iMessage on my iMac. When I send messages on my phone, they don't show up on my computer. iMessages on my iMac says I'm offline and I can't seem to get back online. I closed iMessages and reopened it. I restarted my computer. I logged out then into iMessages on my computer. Any other suggestions?